Whenever some organisms invade our bodies, our immune system responds. They release antigens and antibodies (See: Antibodies) to trap the intruders. Antigens can be foreign antigens or self-antigens. Foreign antigens are substances produced by bacteria, virus, and protozoa. Antigens are molecules that induce immune response and lymphocytes. They are peptides, lipids, and polysaccharides. There is an antigen for every antibody and both of them work together after a tailored response to protect the body. Antigens also match with Ag-specific receptors. Their essential function is to bind with an antibody. They might form in the body or might originate from an external environment. There are different types of antigens including exogenous antigens, endogenous antigens, autoantigens, neoantigens, viral antigens, and tumor antigens. The surface of the antigens have antigenic determinants that are capable of binding with the receptors of lymphocytes. This stimulates the immune system to produce the antibody. They become immunogens and then producing immune response.